The raster scan system used in most television and computer monitor cathode ray tube displays provides horizontal and vertical scanning in what is ideally a generally rectangular linear scan pattern. In most displays, the horizontal scan system runs at a substantially greater frequency than the vertical scan system providing a plurality of sequential vertically displaced horizontal scan lines which fill out a complete raster scan of the display. Several practical considerations and limitations provide obstacles to the completely linear uniform scanning of the cathode ray tube. For example, the basic geometry of cathode ray tube themselves creates geometric distortions in that the faceplate generally defines a radius of curvature which is substantially greater than the distance than the scanned CRT faceplate and the effective center of deflection or bending of the scanning electron beams. This is particularly true of cathode ray tubes used in computer monitors which provide flat or nearly flat display screens. In addition to basic geometry problems found in the cathode ray tube itself, the deflection scanning systems of the display are seldom, if ever, perfectly linear and thus create nonlinearity themselves which contribute to the overall lack of linearity and correct geometry of the raster scan.
These geometry problems give rise to a variety of distortions which, for example, includes the well known pincushion distortion, barrel distortion and trapezoidal or keystone distortion. To correct such problems, practitioners in the art have provided systems which develop geometry correction signals. Such correction signals are provided at either or both the horizontal and vertical scan rates and are used to modify the deflection system scan signals and achieve correction.
For example, the above-mentioned pincushion correction is compensated or corrected by imposing a vertical rate amplitude change upon the horizontal scan drive signal amplitude which is minimal at the top and bottom of the raster and maximum at the center. Barrel distortion is the converse of pincushion distortion and is corrected by applying a vertical rate amplitude change upon the horizontal scan system which is maximum at the top and bottom of the raster and minimum at the center. Similar corrections are derivable for other geometric distortions. In most display systems and in particular the exacting art of computer monitor displays, a single correction is insufficient and a number of corrections simultaneously operative are required to achieve a linear or nearly linear display. Such systems have been provided and have enjoyed different levels of success. However, such correction systems are often complex and subject to their own limitations as to linearity and signal handling capability. Thus, there remains a continuing need in the art for evermore improved and effective geometry correction systems for cathode ray tube displays.
Accordingly, it is a general object of the present invention to provide an improved geometry correction system. It is a more particular object of the present invention to provide an improved geometry correction waveform synthesizer.